This invention relates to display screens, and more particularly, to projection. display screens for rear projection displays, front projection displays and tiled projection displays.
The performance and flexibility of projection displays has increased dramatically in recent years. This has produced dramatic growth in the use of projection displays in an ever increasing number of applications. In addition to film based systems, significant advances have been made in the electro-optic technologies for image generation. These technologies include Cathode Ray Tubes (CRTs), Liquid Crystal Displays (LCDs), electro-mechanical light modulators and numerous other light modulation technologies. To support and complement these advances, screen technology has also advanced.
In its most basic form, a projection display screen may include a light scattering element, or diffuser. The screen may be reflective, in the case of a front projection display, or transmissive in the case of a rear projection display. Numerous variations of light scattering elements have been developed, including volume scatterers, surface scatterers, holographic diffusers, beads, lenticular elements and the like.
While a diffuser can serve the basic function of a projection screen, additional features are often desirable in selected applications. For example, structures that suppress the reflection of ambient light are often incorporated into projection screens. Controlled scattering angles have also been used to maximize the luminance of the viewable light within a range of viewing angles. Uniformity enhancing mechanisms such as Fresnel lenses have also been placed behind or incorporated into the back of rear projection screens.
An illustrative prior art projection system is shown in FIGS. 1A-1B. As shown, a red CRT projector 10, a green CRT projector 11 and a blue CRT projector 12 together project a full color image. The image is focused in the proximity of projection screen 14. Before the light reaches projection screen 14, the light rays are redirected by Fresnel lens 13 to impinge on screen 14 at substantially a normal angle of incidence. As shown in FIG. 1B, screen 14 may be a dual lenticular structure having a rear and front lenticular surface 15 and 16, respectively. The rear lenticular surface 15 approximately focuses the light onto the front lenticular surface 16, in the region between black stripes 18. Black stripes 18 absorb a substantial portion of the incident ambient light, thereby increasing the contrast of the screen. To complete the screen and control the effective scattering profile, diffusers 17 are incorporated into or onto the screen. In the example shown, the additional diffusers determine the degree of scattering in the vertical axis along the direction of the lenslets.
The two lenticular surfaces 15 and 16 function to provide a controlled amount of scatter in the direction normal to the lenticular axes. The lenticular surfaces 15 and 16 may also compensate for the relative offset between the red, green and blue projectors to minimize color shifts as a function of viewing angle.
U.S. Pat. No. 5,434,706 to Matani et al.; U.S. Pat. No. 5,196,960 to Matsuzaki et al.; U.S. Pat. No. 5,457,572 to Ishii et al.; and U.S. Pat. No. 5,724,188 to Kumagai et al. disclose variations on this basic approach. These schemes tend to work well for relatively low resolution applications, such as projection television applications using formats such as NTSC. At higher resolutions, however, the prior art approaches can become obtrusive and can limit the useful resolution of the display. For example. U.S. Pat. No. 5,724,188 to Kumagai et al. discusses some of the difficulties in manufacturing suitable screens for high resolution, including difficulties in making thin screen elements, maintaining rigidity and maintaining contrast.
The difficulties associated with prior art screen technologies, both diffusely scattering and lenticular, become increasingly problematic when the resolution is further increased by providing a number of tiled projectors. In a tiled display, an additional source of non-uniformity is introduced. As illustrated in FIG. 2A, the luminance uniformity of a tiled display is typically a function of viewing angle. More specifically, FIG. 2A shows two non-overlapped projectors 20 and 21 projecting light onto a screen 29. Several resulting scattering profiles 22 through 26 are also shown. Profiles 22 and 26 show how light incident normal to screen 29 is scattered. As the incident light becomes more off-axis, however, profiles such as 23 and 25 result. At the seam, which is the point where the two sets of projected rays meet, a profile such as profile 24 is obtained.
FIG. 2B shows two different observation reference points 27 and 28. From the various profiles, it is clear that the luminance image observed across the screen from reference point 27 is in general different from that observed from reference 28 (i.e. looking at the screen from a different angle). This effect can be reduced by overlapping the images of adjacent projectors, as more fully described in a co-pending U.S. patent application Ser. No. 09/159,340, entitled “METHOD AND APPARATUS FOR PROVIDING A SEAMLESS TILED DISPLAY”, which is incorporated herein by reference. Providing overlap can also aid in other subjective measures of perceived seamlessness, such as color uniformity and vernier mismatch between tiles. Therefore, the ability for a projection screen to support overlap is highly desirable.
Many prior art screens cannot readily support overlap in tiled displays. For example, in a prior art Fresnel field lens approach, little or no overlap is allowed because each projector must typically have a distinct Fresnel lens. The Fresnel lens simply cannot compensate for light emanating from different spaced locations. Because little or no overlap is allowed, the projected image from each projector must typically be precisely matched in size and location with the corresponding Fresnel lens to minimize the visible seams. This greatly impacts the alignment tolerance and stability of the resulting system. Further, it may be difficult to mask slight variations in luminance or color coming from adjacent projectors.
Still further constraints are often placed on high resolution systems. Numerous artifacts such as Moire patterns can result when the screen features interfere with the Fresnel pitch or the projected image pitch. Pixel substructure may be visible in some or all regions of the screen, luminance efficiency may be of high importance and ambient light conditions may be extreme. All of these can lead to significant tradeoffs when working within the framework of prior art screen approaches.